Resumen del producto

The sub-tropical north eastern Pacific is one of the major zones in the ocean where nitrogen is removed by bacterial processes which are enhanced by low oxygen concentrations commonly found in the water column along the Pacific coast upwelling areas. It is well established that the nitrogen isotopic signal (?15N) increases in relation to trophic levels but little is known about the transfer of this ?15N signal from the dissolved fraction to higher trophic levels in oceanic regions with low oxygen. The objectives of this study are: 1) to report ?15N values from different abiotic and biotic components collected in the low-oxygen oceanic region in front of Bahía Magdalena (Mexican subtropical north-eastern Pacific); 2) to compare the ?15N of different trophic levels with analogous organisms in regions where nitrogen fixation is the dominating process, which will allow us to evaluate the actual transfer of ?15N enriched in 15N through the trophic web up to top predators. The ?15N was higher in both abiotic and biological compared to those reported from zones where N fixation is the dominating process. Oxygen concentrations in the oceanic area in front of Bahía Magdalena are low (< 2ml/l) at shallow water depths (< 100m) but not anoxic. Despite this we found that the ?15N signal reflects denitrification and this signal is transferred up though the food web.

Palabras clave: d15N; Subtropical north eastern Pacific; nitrogen cycle; oxygen minimum zone

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